The subject matter herein relates to a voltage level shifter for electronic circuits, such as for translating electrical signals from within an integrated circuit (IC) to outside the IC, where the IC has an internal operating voltage at a different voltage level than an external transfer voltage.
Integrated circuits (IC""s) of today typically operate internally at voltages that are lower than those used in IC""s of just a few years ago. For example, a few years ago, an xe2x80x9cinternal operating voltagexe2x80x9d of about 5.0 volts was common for many IC""s. More recently, an internal operating voltage of about 3.3 volts has become common. Today, internal operating voltages of about 1.8 volts or less have come, or are coming, into use for IC""s.
When two IC""s having different internal operating voltages are to be used together, a voltage level shifter is used to shift one of the IC""s output signals from the IC""s internal operating voltage to that of the other IC. Since newer IC""s are typically designed to be compatible with older, xe2x80x9clegacy,xe2x80x9d IC""s, the voltage level shifter is commonly incorporated into the newer IC""s.
A newer IC may have a lower internal operating voltage at which most of the functions of the IC operate and a higher xe2x80x9cexternal transfer voltagexe2x80x9d at which output signals are transferred to other IC""s. The voltage level shifter, thus, transitions the output signals from the lower internal operating voltage to the higher external transfer voltage. Additionally, input signals are typically shifted by the IC from the higher external transfer voltage to the lower internal operating voltage.
An exemplary prior art voltage level shifter 100, as shown in FIG. 1, receives an input signal VIN 102, typically a digital signal operating at a given clock frequency and voltage level, and produces therefrom an output signal VOUT 104 at the same frequency, but a higher voltage level. The input signal VIN 102 is supplied by internal core IC circuitry (not shown), performing the normal functions of the IC (not shown). The output signal VOUT 104 is supplied to output pins (not shown), which connect to other circuitry or IC""s (not shown), possibly through a printed circuit board (not shown). Additionally, a logic one value for the digital input signal VIN 102 has about the same voltage level as a core voltage VCORE 106, i.e. the internal operating voltage, used to power of the internal core IC circuitry. A logic one value for the digital output signal VOUT 104 has about the same voltage level as an I/O (input/output) voltage VIO 108, i.e. the external transfer voltage, used to power the I/O functions of the IC.
The voltage level shifter 100 includes thick oxide N-MOSFET transistors 110 and 112, thick oxide P-MOSFET transistors 114 and 116 and a thin oxide inverter 118. The sources of the transistors 110 and 112 are connected to ground 120. The drains of the transistors 110 and 112 are connected to the drains of the transistors 114 and 116, respectively. The sources of the transistors 114 and 116 are connected to the I/O voltage 108. The drain of the transistor 110 is also connected to the gate of the transistor 116. The drain of the transistor 112 is connected to the gate of the transistor 114 and also supplies the output signal 104. The inverter 118 is connected to the core voltage 106 and to the ground 120.
The input signal 102 is supplied to the gate of the transistor 110 and to the input of the inverter 118. The inverter 118, powered by the core voltage 106, inverts the input signal 102. The inverted input signal 102 is supplied to the gate of the transistor 112.
Therefore, when the input signal 102 is at a logic zero (i.e. approximately zero volts), the logic zero at the gate of the transistor 110 causes the transistor 110 to turn xe2x80x9coff,xe2x80x9d and the inverted input signal 102 (i.e. logic one) at the gate of the transistor 112 causes the transistor 112 to turn xe2x80x9con.xe2x80x9d Since the transistor 112 is xe2x80x9con,xe2x80x9d the drain of the transistor 112 (and, thus, the output signal 104 and the gate of the transistor 114) is xe2x80x9cpulled downxe2x80x9d to approximately ground, or zero volts or logic zero. The logic zero on the gate of the transistor 114, thus, turns xe2x80x9conxe2x80x9d the transistor 114, so the drain of the transistor 114 (and the gate of the transistor 116) is xe2x80x9cpulled upxe2x80x9d to the voltage level of the I/O voltage 108, i.e. a logic one. The logic one on the gate of the transistor 116, thus, turns xe2x80x9coffxe2x80x9d the transistor 116, so as not to interfere with the logic zero on the drain of the transistor 112 and the output signal 104.
On the other hand, when the input signal 102 is at a logic one (i.e. the internal operating voltage), the logic one at the gate of the transistor 110 causes the transistor 110 to turn xe2x80x9con,xe2x80x9d and the inverted input signal 102 (i.e. logic zero) at the gate of the transistor 112 causes the transistor 112 to turn xe2x80x9coff.xe2x80x9d Since the transistor 110 is xe2x80x9con,xe2x80x9d the drain of the transistor 110 (and, thus, the gate of the transistor 116) is xe2x80x9cpulled downxe2x80x9d to approximately ground, or zero volts or logic zero. The logic zero on the gate of the transistor 116, thus, turns xe2x80x9conxe2x80x9d the transistor 116, so the drain of the transistor 116 (and, thus, the output signal 104 and the gate of the transistor 114) is xe2x80x9cpulled upxe2x80x9d to the voltage level of the I/O voltage, 108 (i.e. a logic one at the external transfer voltage). The logic one on the gate of the transistor 114, thus, turns xe2x80x9coffxe2x80x9d the transistor 114, so as not to interfere with the logic zero on the drain of the transistor 110 and the gate of the transistor 116.
With the lower internal operating voltages and higher clock frequencies coming into use with many IC""s, the thick oxide N-MOSFET transistors 110 and 112 cannot perform adequately. The internal operating voltages, for example, are becoming so low that they are approaching the xe2x80x9cthreshold voltagexe2x80x9d of the transistors 110 and 112. The threshold voltage of a transistor is the minimum voltage that can be applied to the gate of the transistor to activate the transistor. Therefore, if the internal operating voltage (i.e. the logic one voltage of the input signal 102) becomes as low as the threshold voltage of the transistors 110 and 112, then the transistors 110 and 112 cannot be activated and the voltage level shifter 100 will not operate. Additionally, if the logic onand voltage level of the input signal 102 is relatively larger than the threshold voltage of the transistors 110 and 112, then the transistors 110 and 112 can be turned on relatively fast. However, if the logic one voltage level of the input signal 102 is relatively close to the threshold voltage of the transistors 110 and 112, then the transistors 110 and 112 will switch from xe2x80x9coffxe2x80x9d to xe2x80x9conxe2x80x9d relatively slowly. In this case, the transistors 110 and 112 cannot be activated quickly enough for the desired clock frequency of the IC, and the voltage level shifter 100 will not operate.
It is with respect to these and other background considerations that the subject matter herein has evolved.
The subject matter described herein involves an integrated circuit (IC) having a voltage level shifter capable of operating with the lower internal operating voltages and higher clock frequencies used by current and upcoming IC""s. The transistors (i.e. xe2x80x9cswitching transistorsxe2x80x9d) within the voltage level shifter that are activated, or xe2x80x9cswitched on and off,xe2x80x9d by the internal operating voltage of the IC incorporate a thinner oxide than in the prior art. Therefore, the threshold voltage of the switching transistors is lower than the thicker-oxide transistors in the prior art, so the internal operating voltage required to turn xe2x80x9con,xe2x80x9d or xe2x80x9cactivate,xe2x80x9d the switching transistors is lower than the voltage required to turn xe2x80x9conxe2x80x9d the transistors in the prior art. Additionally, the frequency at which the switching transistors can be switched xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d at this voltage is higher than that for the transistors in the prior art at this voltage.
Also, the switching transistors have a maximum voltage below which they can operate that is less than the external transfer voltage of the IC. Therefore, additional transistors are placed in the current path between the switching transistors and the I/O voltage source in order to limit the voltage drop across the gate oxide (i.e. from the drain to the source) of the switching transistors. The reduced voltage across the gate oxide prevents the switching transistors from xe2x80x9cfailingxe2x80x9d due to the presence of the external transfer voltage on the drain of the switching transistors.